The present invention relates to a noise reduction circuit for use in television receiving equipment to reduce noise included in a luminance signal among television signals.
A noise reduction circuit disclosed in Japanese Patent Laid-open (kokai) No. 62-122380 corresponding to U.K. Pat. No. 2179820 (hereinafter referred to as the cited reference) extracts a high-frequency component including noise from an image signal with a high-pass filter, limits the amplitude of the high-frequency component, inverts the phase of the high-frequency component, and then adds the high-frequency component to the original signal to reduce the noise.
Generally, a circuit as shown in FIG. 1 of the cited reference is designated as a coring circuit. As is known from FIG. 2, which shows the input-output characteristics of the known coring circuit, the output of the coring circuit remains constant for an AC cut input in the range of .+-..DELTA.V/2 with respect to the origin. The coring circuit extracts minute variations from the input signal. The value .DELTA.V is designated as a coring range.
In FIG. 1 of the cited reference, a circuit comprising an amplitude limiter B, a phase inverter -V and an adder 3 corresponds to the coring circuit.
FIG. 1 shows a conventional noise reduction circuit, which has a low-frequency amplifier in addition to the circuit disclosed in the cited reference. This noise reduction circuit comprises an input terminal Y.sub.in to which a luminance signal is applied, a high-pass filter 3 connected to the input terminal Y.sub.in, a coring circuit 4 connected to the high-pass filter 3, a high-frequency amplifier 5 connected to the coring circuit 4, a low-pass filter 6 connected to the input terminal Y.sub.in, a low-frequency amplifier 7 connected to the low-pass filter 6, an adder 8 connected to the high-frequency amplifier 5 and the low-frequency amplifier 7, an output buffer 9 connected to the adder 8, and an output terminal Y.sub.out. A coring gain setting circuit 1 for setting a coring range is connected to the coring circuit 4, and a gain setting circuit 2 is connected to the high-frequency amplifier 5. The high-pass filter 3 filters out the high-frequency signal component of a luminance signal applied to the input terminal Y.sub.in, and the low-pass filter 6 filters out the low-frequency signal component of the luminance signal. The high-frequency signal component is applied to the coring circuit 4. Suppose that luminance signal as shown in FIG. 3(b) consisting of an original luminance signal as shown in FIG. 3(a) and a noise signal is applied to the input terminal Y.sub.in. Then, the low-pass filter 6 provides an output signal as shown in FIG. 3(c), while the high-pass filter 3 provides an output signal as shown in FIG. 3(d). Since most noise is a high-frequency signal, the noise signal is included in the output signal of the high-pass filter 3. The coring circuit 4 cores the output signal of the high-pass filter 3 to provide an output signal as shown in FIG. 3(e) obtained by reducing the high-frequency noise signal. Fundamentally, noise signal components outside the coring range .DELTA.V shown in FIG. 3(d) cannot be cored by the coring circuit 4 and are included in the output signal of the coring circuit 4. However, since such noise signal components are very small, the same are not shown in FIG. 3(e). Since the amplitude of the high-frequency signal component of the input image signal is reduced in coring the same by the coring circuit 4, the high-frequency amplifier 5 amplifies the output signal of the coring circuit 4 to the original level to provide an output signal as shown in FIG. 3(f). The output signal of the high-frequency amplifier 5 is applied to one of the input terminals of the adder 8. On the other hand, the low-frequency signal component as shown in FIG. 3(c) filtered out by the low-pass filter 6 is applied to the other input terminal of the adder 8 after gain adjustment. The adder 8 adds the high-frequency signal component and the low-frequency signal component to provide an output signal as shown in FIG. 3(g) at the output terminal Y.sub.out.
The coring range .DELTA.V of the coring circuit 4 is decided uniquely by a setting voltage provided by the coring gain setting circuit 1. When the coring range is increased excessively to increase noise reduction, part of the high-frequency signal component of the input image signal is eliminated to deteriorate the frequency characteristics. On the other hand, when the coring range is decreased excessively, the effect of noise reduction is insufficient.